In the past, renewable energy sources, i.e. energy sources that can be replenished in a relatively brief time period, such as biomass or other cellulosic materials, accounted for about 90% of the energy consumed in the United States. By more recent times, however, the United States had become, and continues to be, heavily reliant on the combustion of non-renewable fossil fuels, such as coal, natural gas, and oil, to meet its growing energy needs.
Coal is the most abundant fossil fuel in the United States, and more than half of the electricity produced in the United States comes from coal-fired power-generating plants. Though coal is presently abundant in the United States and is, on an energy-per-Btu basis, currently the least expensive of the fossil fuels, it is nonetheless a non-renewable energy source and will likely become an increasingly scarce commodity over the next two centuries. Coal combustion also contributes significantly to several environmental pollutants, including heavy metals, such as arsenic and mercury, and radioactive metals, such as uranium and thorium. Furthermore, coal combustion is the source of a substantial percentage of United States emissions of sulfur dioxide, an agent implicated in the formation of acid rain.
Coal exists in a variety of grades, each with a characteristic moisture, ash, and sulfur content and an associated heat value, generally in the range of 4,000 to 15,000 Btu/lb. The grindability of a particular coal or other materials is the measure of its resistance to crushing that depends on several factors, including the moisture content, ash content, and the physical integrity of the coal. Knowing the grindability of coal is important because coal that is difficult to grind can limit the capacity of a pulverized coal-fired boiler by reducing the throughput of the coal pulverizers and because it has implications for the transportation and storage of the coal.
Biomass, a renewable energy source that contains energy stored from solar radiation, includes a wide range of organic materials produced by both animals and plants. Fuel pellets made from biomass are increasingly of commercial interest due to concerns about the finite world-wide reserves of coal and other fossil fuels, as well as the significant environmental damage resulting from coal combustion pollutants. Interest in methods of combining renewable energy sources with non-renewable waste materials that can be recycled or reused beneficially, such as waste plastics that would otherwise end up in landfills, has been driven by cost as well as environmental concerns as it becomes increasingly expensive to site, obtain operating permits for, and maintain landfills.
Previous methods of producing fuel pellets can have a variety of drawbacks, for commercial and/or industrial uses, including lower heating value and/or poor handling characteristics (e.g., a tendency to crumble), thus rendering the pellets unsuitable for many coal-fired furnaces or industrial boiler applications.